This invention relates to an apparatus and method for drying fibrous web material such as a wet paper web.
A number of systems have been devised for drying paper webs and the like, perhaps the most common being those wherein the web is pressed or otherwise applied to heated rolls such as a Yankee dryer which is a steam heated cylinder of large diameter.
U.S. Pat. No. 4,324,613 issued to Wahren discloses a variation of the heated cylinder approach wherein drying of a web is effected by transferring heat very rapidly, directly to a web under high pressure. Heat is directly supplied to the outer surface of a roll and the heated surface is then pressed under high pressure against the moist web to consolidate and dry the web.
U.S. Pat. No. 4,324,613 specifically suggests that the external heating of the roll may be accomplished through the use of liquid, pulverized, or gaseous fuel.
The system of U.S. Pat. No. 4,324,613 utilizes a technique known in the art as "impulse drying" wherein temperatures and rates of thermal energy flow used for the heated roll are so high as to cause a very rapid, violent and almost explosive generation of steam to take place at the interface between the roll and the moist web. The steam thus formed tends to pass straight through the web, carrying with it any free water remaining in the cavities between the fibers of the web.
There are limitations and operational difficulties inherent in a dryer system such as that taught by U.S. Pat. No. 4,324,613 wherein heat is applied directly to the outside of a dryer roll. For example, use of an external combustion approach results in obvious environmental problems. Further, the application of heat directly to the outer surface of the dryer roll can result in deterioration or contamination of the very surface to be contacted by the paper. An exterior burner system must also be protected from water and vapor contained in the web.
Perhaps the most profound difficulties, however, result from the fact that use of an external heating source necessarily means that the dryer roll cannot be heated about its entire periphery. Thus, heat flux is not as great or effective as it would be if application of heat took place about the full peripheral area of the roll. In addition, the application of heat about its partial periphery can cause warping of the roll.